You really need to add the reference ranges to each.
But my comments will be based on what we frequently see for normal ranges.
First off. The antibody figures it is clear you have Hashimoto's. This is fairly common and is an autoimmune condition where the body thinks your thyroid gland is a foreign invader and the antibodies go and attack and sometimes rapidly, and other times slowly over time "kill" your thyroid.
While your FT4 is low, and I would NORMALLY say should be higher in the range. Your FT3 is much higher than I would have expected with such a low FT4 value. As your thyroid gland does produce some T3, it mostly produces T4 and the T4 must convert into T3 to ultimately be used at the cellular level. And this is exactly why it is so important to have the Free T3 level checked.
Brittle nails is normally a sign of Hypo, as is fatigue. Again normally.
So that is what is causing you to be hypo (low) thyroid.
TSH. This is virtually a worthless test and screening test at best. And should NEVER be used to adjust dosage and even more useless when a person is on thyroid replacement.
It is not all that uncommon to alternate dosage of T4 medication. Adding up the total Mcg per week you get 450 mcg total. That averages out to 64 mcg per day. You could also get approximately the same dosage by taking one 50 mcg and a second 12.5 mcg pill (or split a 25 mcg in half). That would give you 62.5 mcg a day.
it is a bit odd of the only the weekend for two days at double the dosage (100 rather than 50) of the rest of the week. while the T4 level is basically an average of T4 dosage over about 6 weeks. I would personally rather take a more consistent dosage as I outlined above, or at least want to spread out the 100 mcg dose more than just the weekend. Although it is easy to remember as weekend only. Maybe it is just me. But I would personally take one of the two 100 mcg on say Wed, and the other 100 mcg on either Saturday or Sunday.
Others may chime in with different opinion.
But please list the reference ranges and any other symptoms you have other than brittle nails and fatigue.
I find it very unusual to have such a bottom of bucket FT4, yet your FT3 is at, or above the top of the range.
Yes peeling nails was one of my symptoms of hypothyroidism. I found an article from Oak Creek Relied and Wellness Six Patterns of Hypothyroidism. Pattern 4 seems to be your issue if you have higher T3 than T4.
"Pattern 4: Over Conversion
Pattern 4 is the opposite of pattern 3. The thyroid and pituitary are again working properly, but this time there is too much T4 converted to T3 (remember roughly 20% of T4 is conversted into reverse T3 which is inactive and excreted). High levels of testosterone is the cause which also has the added effect of decreasing the amount of thyroid binding globulin (TBG: the taxi that carries the thyroid hormones).
The high levels of T3 in conjunction with low TBG causes cells to become resistant to T3. They close the doors and do not allow anymore T3 into the cells. TSH levels will be normal even though the patient is showing signs of hypothyroidism
This pattern is similar to insulin resistance. In fact, patients with thyroid over conversion often also present with insulin resistance and polycystic ovary syndrome (PCOS) which is also caused by increased testosterone levels. Managing blood sugar is extrememly important. Since there is already too much T3, prescription thyroid medication will not help in this pattern."
I just came from the dermatologist who told me my peeling, splitting nails were most likely a sign of biotin or vitamin D deficiency. Biotin isn't routinely tested, but you can take the daily recommended dosage without harming yourself, or you can take a B-complex that contains the daily recommended dosage of biotin.
Vitamin D is a test that any doctor can order for you; in fact, it's one that should be ordered routinely, because vitamin D is necessary for the proper metabolism and synthesis of thyroid hormones.